1. Field of the Invention
This invention pertains generally to a method and apparatus for manufacturing a mechanical seal for use in sealing liquids in fluid handling applications. More specifically, seal faces are manufactured in such a way that they have improved properties of flatness, resistance to deformation, etc.
2. State of the Art
Mechanical seals are used to seal liquids in fluid handling applications. The seals are manufactured from a variety of metals and ceramic materials. One of the most versatile and therefore the most common material used in seal assemblies is cemented tungsten carbide.
One of the most common applications of mechanical seals is for sealing shafts on pumps where it is desirable to prevent the passage of a fluid past the shaft. It is also the case that the many types of pumps and media (fluid) being pumped require a large range of sizes and types of seals. Accordingly, seal faces typically range in diameter from less than an inch to greater than eighteen inches, with two to six inches being the most common diameters.
The sealing of the media being pumped is accomplished by rotating two seal faces against each other. Because the seal faces are extremely flat (a fraction of a micrometer in variance) the media generally cannot pass through the gap between the seal faces. Typically, one seal face will be stationary (the static face) and the mating face will be driven (the rotating face) at shaft RPM. In order to produce and maintain this small gap the seal faces must exhibit very tight control of flatness, deformation, thermal expansion, thermal conductivity, corrosion, and wear resistance. These factors will each be explained so as to understand the specific problems that they pose to the performance of a seal.
Regarding flatness of the seal faces, precision grinding and lapping using abrasive compounds is used in the manufacturing process thereof. The standard call-out for flatness is two Helium light bands, while some companies require a higher tolerance of only 1 Helium light band. Flatness is measured using a monochromatic light reflected onto an optical flat, as is known to those skilled in the art. When the seal face is placed on the optical flat a series of light bands will be produced. A thin line placed parallel to these bands should not cross more than the number of light bands in the call-out. If the seal face is not flat, the light bands will exhibit one or more patterns such as curvature, high spotting, etc. The light band pattern is analogous to contour lines on a topographical map. One Helium light band is equal to a flatness measurement of 17.4 millionths of an inch.
Along with flatness, deformation is another important attribute that must be controlled. Deformation is caused by the seal face being subjected to mechanical forces. Generally, it is controlled by the nature of the material used in fabricating seals. In other words, it is the nature of cemented tungsten carbide's extreme rigidity that enables the seal to resist deformation. For example, cemented tungsten carbide is one of the most rigid materials available, being 3 times more rigid than steel. This rigidity allows for seal designs with very small cross sections. One result of this rigidity is that seal assemblies are very compact.
Thermal expansion of the seal is another factor that must be controlled in order to resist deformation. When thermal expansion causes deformation of the seal faces, the consistent gap therebetween is eliminated. Advantageously however, cemented tungsten carbide provides a low coefficient of thermal expansion, thereby minimizing the possibility of deformation. Furthermore, a low coefficient of thermal expansion also provides advantages in conditions that subject the seal face to thermal shock such as dry running.
Seal performance is often measured in terms of the pv value, where p pressure and v=peripheral velocity. This combination of sealing pressure and velocity produces high temperatures that can vaporize the liquid film created by the sealed-out medium. High thermal conductivity of a seal enables it to conduct excess heat away from the seal faces before high temperatures can build to a dangerous level. Therefore, high thermal conductivity in the seal face material is critical in limiting this type of problem.
For example, a first cemented tungsten carbide seal face which is in contact with a second cemented tungsten carbide seal face in a water seal application can work without a problem at pv values of 100 bar m/s, with a maximum pv value of 500 bar m/s. However, higher pv values can be obtained by running a cemented tungsten carbide seal face against another material, such as carbon.
Another problem that a seal face must contend with is that of corrosion. When corrosion attacks the seal face, it causes pitting, leaching, etc., which can lead to seal failure. In the prior art, corrosion resistance is controlled by utilizing special binders in the cemented tungsten carbide. The most common binder used in seal faces to resist corrosion is Nickel. The standard percentage of Nickel binder used in the United States for seal faces is 6% by weight. However, the best possible solution to corrosion would be no binder whatsoever.
The most widely recognized property of cemented tungsten carbide is wear resistance. Wear resistance is especially critical in seal applications where the medium being sealed contains hard particles that can penetrate between the seal faces. Cemented tungsten carbide offers the wear resistance required to combat this type of problem while exhibiting higher thermal conductivity than ceramics which provide even higher wear resistance than cemented tungsten carbide. Disadvantageously, ceramics are also extremely brittle, less able to withstand thermal and mechanical shock, and require special handling.
Cemented tungsten carbide is recommended for seal faces when the application requires a material with some or all of the properties of flatness, deformation, thermal expansion, thermal conductivity, corrosion, and wear resistance as discussed above. In light duty applications or in applications requiring extreme corrosion resistance where there is little or no mechanical/thermal shock, silicon carbide, bronze, carbon, cast iron and stainless might also be used.
Other limiting factors when choosing cemented tungsten carbide as a seal material are size, complexity, and cost. The size of a seal ring can eliminate the use of cemented tungsten carbide, as most domestic suppliers have a limit of roughly 13" diameter. Complex seal faces with geometric features that are difficult to press and/or machine, geometries not suited to carbide (very thin walls, internal grooves perpendicular to the face, etc.) will also eliminate the use of carbide. Cost is the final factor when considering cemented tungsten carbide as a seal material. In some cases the product can be produced from cemented tungsten carbide but due to size and/or complexity, alternate materials that will perform in the application at lower cost are chosen.
Nevertheless, when cemented tungsten carbide is appropriate as a seal material, the manufacturing processes of the state of the art are the same as when manufacturing other carbide products (i.e. turning inserts, wear parts, punches, dies, etc.). Specifically, tungsten carbide powder, where each crystal has a covalently bonded chemistry of Tungsten and Carbon and is not a mixture of the two, is mixed with Cobalt or Nickel (referred to as the binder) powder for the desired binder composition. This mixture is then cold pressed to the desired shape and is referred to as a pre-sintered blank. The blank has the consistency and appearance of black chalk. The shape is generally pressed 14 to 19% larger than the finished part shape. Sometimes wax is mixed in with the binder so that the pre-sintered blank can be machined to more complex geometries without breaking, fracturing or chipping. The pre-sintered blank is then placed in a sintering furnace at a high enough temperature to melt the binder.
Many companies are now using a sinter HIP (Hot Isostatic Press) to generate high pressures during the sintering process to eliminate pits and voids in the carbide. When the blank is removed from the sintering furnace it is now referred to as a sintered blank. Because the binder has melted, the blank size has shrunk from its original size to the approximate size of the finished part. However, because of the shrinkage, the sintered blanks must have carbide stock which is generally removed by grinding for the part to achieve its finished size. On small production runs of carbide (the cemented tungsten carbide), 0.030" of stock per side is allowed for finishing, while on large production runs in controlled manufacturing conditions, 0.010" stock is allowed.
The state of the art method used to machine cemented tungsten carbide into a seal is by grinding. There are many types of grinding machines used to grind stock from carbide parts. Generally, 0.0005" to 0.002" can be removed per pass of a grinding wheel. Diamond grinding wheels are used almost exclusively because of cemented tungsten carbide's hardness. If more than 0.002" stock is removed per pass with a grinding wheel, the risk of surface cracking and heat checking increases dramatically. Creep feed grinders are also used to remove large amounts of cemented tungsten carbide. The wheel is fed slowly into the part, but only one pass is used.
Now that the methods of grinding cemented tungsten carbide have been described, it is important to realize that there are some notable problems with the state of the art. For example, grinding leaves surface damage and residual surface stresses. Applications that require the seals to be subjected to high loads, fatigue resistance, corrosion, resistance, etc., require a damage free surface on the seals to prevent crack initiation at the surface. Therefore, feeds of the seals are relatively slow and grinding depths are limited. Furthermore, inside diameters of the seals take longer to grind because the wheel is smaller and grinds at a slower surface speed. Because stock is removed by a grinding wheel, finished part geometries can only be as complex as a grinding wheel geometry will allow. Disadvantageously, cemented tungsten carbide rings (such as those used to form large seals) often warp from the residual grinding stresses and require that the part be scrapped or reworked at additional cost.
It would therefore be an improvement over the state of the art in carbide seal manufacturing for fluid applications to be able to manufacture the seal having a surface with less damage than can otherwise be created by grinding. It would be another improvement if the seal surface had reduced residual surface stresses. It would be a further improvement if it was possible to form an edge between a top surface of the seal and an outer side edge that had less damage than when created by grinding, to thereby better inhibit crack formation. Other improvements would be to decrease the manufacturing time, enable the manufacturing of more complex finished geometries than are possible with grinding, and improve the ability to make larger seals without the danger of warping that is caused by residual grinding stresses.